A silicone gel has a low elastic modulus and excellent mechanical strength and is also excellent in terms of heat resistance, cold resistance, and the like. Therefore, it has been used in a wide range of fields, including adhesives for various electrical/electronic parts, sealing agents, potting materials, coating materials, damping materials for use in optical pickup devices, and the like.
As a damping material for use in an optical pickup, a specific additional reaction-curable silicone gel has been proposed as described in Patent Document 1. However, there is a problem of low productivity because of the heated reaction. When a mercaptoalkyl group-containing polyorganosiloxane as described in Patent Document 2 is used, curing proceeds more rapidly upon irradiation with light. However, there is a problem in that the cured product undergoes softening degradation when exposed to high temperature and high humidity for a long period of time. That is, the penetration value increases, whereby the damping performance deteriorates or the cured product flows out. Patent Document 3 proposes a photocurable organopolysiloxane obtained by a reaction of an organopolysiloxane containing a terminal amino group with a (meth)acrylic group-containing isocyanate. However, there also is a similar problem in that it undergoes softening degradation when allowed to stand at high temperature and high humidity.
As the sealing agent, Patent Document 4 discloses that a sealing agent containing a photocurable gel, which is a photocurable organopolysiloxane obtained by a reaction of an organopolysiloxane containing a terminal amino group with a vinyl group-containing isocyanate, is used for mobile phones, waterproof watches, waterproof miniature switches, waterproof small resistors, hard disk covers, etc. However, the composition disclosed therein is not suitable for sealing applications for use at high temperature and high humidity. This is because this composition undergoes softening degradation at high temperature and high humidity, causing a problem of decreased sealing performance.